1. Field of the Invention
This invention relates to a digital control method and device for an internal combustion engine and more particularly to improvements in a digital control method and device suitable for use in an engine of a motor vehicle having an electronic fuel injection device, wherein the engine is digitally controlled in response to engine condition signals that have been read out from sensors for detecting the conditions of the engine.
2. Description of the Prior Art
Heretofore, there has been put into practical use a digital control method for an internal combustion engine, wherein various sensors including a coolant temperature sensor, an intake air temperature sensor, an intake air pressure sensor, an intake air flow rate sensor, an engine rotation sensor and the like for detecting the conditions of the engine are provided on various portions of an internal combustion engine such as motor vehicle engine or the like, and fuel injection time of fuel injected into a combustion chamber of the engine, ignition timing, idle rotation speed and the like are digitally controlled in response to the engine condition signals including an engine coolant temperature, an intake air temperature, an intake air pressure, an intake air flow rate, a crank angle and the like which have been read out from the aforesaid sensors. In a digital control device for performing the aforesaid digital control, normally, an analogue signal from each sensor, for example, a coolant temperature sensor 10 for detecting an engine coolant temperature is, as shown in FIG. 1, converted into a digital signal by an Analogue-Digital Converter (hereinafter referred to as an "A/D converter") 14 in a digital control circuit 12, and thereupon, taken into a Central Processing Unit (hereinafter referred to as "CPU") 16 for performing various calculating operations. More specifically, a voltage from a battery 22 is applied to the aforesaid coolant temperature sensor 10 through a voltage regulator 18 having an output voltage Vc of 5 V, for example, and a resistor 20, whereby a change in voltage by a change in resistence of the coolant temperature sensor 10 due to a change in the engine coolant temperature is taken into the A/D converter 14 as an analogue signal indicating the engine coolant temperature.
The above-described digital control method features that the engine is accurately controllable. However, heretofore, a signal from each sensor, for example, a coolant temperature signal read out of an output from the coolant temperature sensor 10 has been taken in at a constant cycle of 1 or 2 seconds and converted into a digital signal in the A/D converter 14, without synchronizing with the rotation of the engine. Hence, for example, when a power supply voltage from the battery 22 drops greatly and the supply voltage Vc of the coolant temperature sensor 10 also drops during start and the like, an output voltage from the coolant temperature sensor 10 also drops. Thus, for example, when a thermistor type temperature sensor is used as the coolant temperature sensor 10, a temperature higher than the actual temperature is detected. As the result, there have been cases where too short fuel injecton time during start results in start failure, an insufficient increase after the start leads to unsatisfactory drivability, and, worst of all, an engine stall is caused.